New solid state NMR methods for determining the backbone conformations of peptides and proteins at specific, isotopically-labeled sties have been developed and applied to problems in biophysical chemistry and structural biology. The new methods are of two types. Both methods have been applied principally to peptides that are labeled with carbon-13 at two successive carbonly sites in the backbone. The first method, called two-dimensional magic-angle-spinning (2D MAS) NMR exchange spectroscopy, provides structural information that is entirely angular in nature. The second method, called constant-time double-quantum excitation (CTDQE), provides structural information that is primarily in the form of internuclear distances. Solid state NMR data obtained with either method can be analyzed in terms of the f and y dihedral angles that define the peptide backbone conformation, using data simulation and analysis software that we have developed. The new methods have so far been used in two separate studies. The first study is an investigation of the conformation of a 17-residue helix-forming peptide, intended to address a current issue in biophysical chemistry regarding the precise nature of helical structures adopted by short peptides. We find that this peptide exists almost exclusively as an a-helix in frozen glycerol/water solutions, but that it converts to a roughly equal mixture of a-helices and 3/10-helices when 5 M urea is added to the solutions. This is a surprising result, because it is commonly assumed that urea would simply convert a-helices to random coils. The second study is an investigation of the conformation of a 24-residue peptide derived from the V3 loop of the HIV envelope glycoprotein gp120 when bound to Fab fragments of an anti-gp120 antibody. We find that a conserved GPGR motif in the middle of the V3 loop peptide does not adopt a type IIb turn that had been predicted from sequence analyses by several other groups and that was believed to play a role in antibody recognition. Together, these two studies demonstrate the utility of solid state NMR methods in the structural characterization of partially-disordered biopolymers and of complexes that are of real biological complexity.